Gwent card art
Hemdall is a mythical hero. According to the beliefs of the inhabitants of Skellige, there is a magical golden rooster known as Kambi whose task it is to awaken Hemdall before the final battle of good and evil, known as "Ragh nar Roog". Hemdall is then said to answer that call and fight the forces of Chaos, and presumably prevail.
Excerpt from game files
Belief in the mythical snake Uroboros is widespread throughout all of Skellige. Winding itself up into a figure eight, the snake took its own tail between its teeth and in so doing created time. The circle of time will be complete when Uroboros once again bites its tail and closes the loop. This will commence the so-calledFinal Age, Tedd Deireadh, also called the Time of the White Frost or the Wolven Blizzard. It is thought many signs will foretell its coming - sources mention a wolf eating the sun and an eclipse of the moon. Next there will be a battle known as Ragh nar Roog - the Last Battle, the Battle of Light and Dark. It will begin with the appearance of a longship made of the claws of dead men and carrying an army of wraiths and demons. Standing watch for the coming of the longship in his hunting blind is a hero named Hemdall. The golden cockerel, Kambi, will warn of the longship's arrival, and then Hemdall will mount the Rainbow Bridge, the Bifrost, to stop the invading powers of darkness. As he goes to fight, Hemdall will sound his horn, and for Skelligers this will be the call to take up arms and join the Last Battle. This final conflict will decide whether eternal night will fall upon the world or a new dawn will rise.
A local legend recounts how all clans in Skellige can trace their lineage back to Hemdall. Supposedly one summer solstice he left his hunting blind and spied Heulyn, a goddess. They fell hopelessly in love and spent the entire summer together, yet when autumn came, Heulyn had to depart. That is why the summer months, when Heulyn stays with Hemdall in Skellige, are the most fortunate days in the Isles.
Six sons were born of Hemdall and Heulyn's love, giving rise to the six clans of Skellige. Their father let them take one isle each, but before they did so, they had to prove their manliness in a fight for living space, by rooting out the aboriginal inhabitants or coming to an understanding with them. That is how the sons of Hemdall and Heulyn first gained renown, and the memory of their deeds is honored by their descendants, the members of the clans they founded.
It appears that Hemdall is loosely based on Heimdallr, a god from Norse Methology.